Air Conditioning Compressors - Delphi

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Th e p r e fe rred c o n f i g u r ation fo r a piston c o m p r e s s o r s o o n m ove d t o a d o u ble acting s wa s h - p l a t e d e s i g n o f w h ich t h e Harrison ( D e l p h i ) H D 6 i s t y p i c a l . Th e fixe d swash- p l a t e i s h e l d b e t we e n t h e o p p osing ends o f t h e d o u b l e p i s t o n by a pair o f s hoes and t wo s t e e l balls. S e c t i o n t h r o u g h a n e a r l y f i xe d s wa s h-plate c o m p r e s s o r, t h e D e l p h i H D 6 A s e c o n d p o s i t i ve d i s p l a c ement c onfigura- t i o n u s e d i s t h e s l i d i n g va n e . I t o ffers ad- va n t a g e s o f c o m p a c t n e ss a n d l ow cost but c a rries t h e p e n a l t y o f ve r y p o o r e fficiency at h i g h p r e s s u r e r a t i o s . Th e o t h e r m a i n c o n f i g u r a tion i s t he scroll c o m p r e s s o r i n w h i ch o n e s c r oll w i th an in- vo l u t e s u r fa c e o r b i t s i n side a fixe d second s c r o l l o f t h e s a m e s h a pe. I t can be seen f r o m t h e f i g u r e t h a t t h e c o n t a c t points of t h e scrolls enclose a cav it y of reducing size ( s h a d e d green) that move s from the outside t o t h e c e n t r e a s t h e o n e scroll o r b i ts inside t h e o t h e r. Th e r e s u l t i s a n i n d u c tion of gas a t t h e p e r i p h e r y o f t h e scroll a n d deliver y o f c o m p r e s s e d g a s a t t h e c e n t r e. These c o m p r e s s o r s a r e ve r y e ff icient a nd quiet b u t r e p r esent a s i g n i f icant ch allenge for m a n u fa c tu r e a n d h e n c e fo r c o s t. They have t h e d i s a d va n t a g e t h a t a l t h o u g h a variable ve r s i o n o f t h i s c o m p r e s sor i s p o ssible, the s u p e r i o r e ff i c i e n c y a d va n t a g e is lost. I l l u s t r a t i o n o f Scroll compression process Early compressors were all fixed displacement and system control was achieved by switching the compressor on and off by means of an electro-mechanical clutch cont ained within the compressor pulley. As consideration began to be given to the application of air conditioning to smaller vehicles with smaller engines it was found that the significant and sudden increase in engine load that resulted from compressor switching caused problems for the small engine with result ant issues of drivabilit y. A compressor with variable displacement offered the solution to this problem. In 1985 Harrison (<strong>Delphi</strong>) were the first to go into series production with a variable compressor, the V5, a wobble plate design with five pistons. Th e m e ch a n i s m o f a va r i a b l e wo b b l e p l a t e c o m p r e s s o r Although scroll, vane and piston compressors are all still in use in automotive applica- tions, the piston compressor is dominant. Where the combination of efficiency and controllabilit y is the key requirement, piston compressors offer the best solution. P i s t o n C o m p r e s s o r s The compressor has the t ask of inducing gas at a low suction pressure and delivering it at a higher discharge pressure. It achieves this by using reed valves that operate automatically to control the fluid flow into and out of the cylinders. St arting with the piston at top dead centre the pressure in the small clearance volume will be close to the discharge pressure as the piston has just delivered its charge. As the piston recedes and the pressure in the clearance volume falls, the delive- r y reed valve closes automatically, driven by the discharge pressure. Since the clearance
vo l u m e i s s m a l l , t h e p r e s s u r e d r o ps quickly u n t i l i t i s b e l ow t h e s u c tion p r e ssure. At t h i s s t a g e t h e i n t a ke valve o p e n s and the f u r t h e r m ove m e n t o f t h e p iston induces f r e s h ch a r g e . Cylinder Pressure - bar 25 20 15 10 5 0 suction pressure P s discharge pressure P d 5 10 15 20 Cylinder volume - cm 3 I n d i c a t o r d i a g r a m fo r s wa s h - p l a t e c ompressor A ft e r b o ttom d e a d c e n t re t h e p iston com- p r e s s e s t h e g a s i n d u c e d i n t o t h e cylinder u n t i l i t r e a ch e s t h e d i s charge p r essure. At t h i s s t a g e t h e d e l i ve r y valve i s o p e ned auto- m a t i c a l l y by t h e p r e s s u r e d i ffe r e ntial and t h e c o m p r e s s e d ch a r g e is delive r ed. Th e r e a r e s eve r a l key fe a tu r e s o f this ove - r a l l p r o c e s s i m p o r t a n t for p e r fo rmance and e ff i c i e n c y. Th e c l e a r a n c e volume must be as s m a l l a s p o s s i b l e w i t h o u t risking interfe - r e n c e b e t we e n t h e p i s t on a n d t h e cylinder h e a d a s a r e s u l t o f d i ffe r e n tial t h e r mal expan- s i o n o r s t r e s s e ffe c t s . Th e c o nsequences o f f i n i t e c l e a r a n c e vo l u m e a r e p rincipally i n d i c a t e d by t h e c o m p r essor volumetric e ff i c i e n c y w h i ch i s t h e ratio o f t he volume o f d e l i ve red g a s m e a s u red a t t h e suction p r e s s u r e relative t o vo l ume displaced by the p i s t o n . Th e d y n a m i c r e s p o nse characteris- t i c s o f t h e r e e d va l ve s are a lso i mport ant. Vo l u m e t r i c e ff i c i e n c y i s a lso impacted by the q u a n t i t y o f g a s t h a t l e a k s p ast t he pistons t o t h e c r a n k c a s e , w h i ch is d e t e rmined by p i s t o n s e a l i n g a n d c l e a r a nces. E ff i c i e n c y D e f i n i t i o n volumetric η v isentropic η s mechanical η m Ta b l e o f c o m p r e s s o r e ff i c i e n c i e s delivered volume displaced volume ideal minimum work needed work done to compress gas work done on compressed gas work input to compressor shaft The second measure of compressor performance qualit y is the isentropic efficiency. It is given by the ratio of the theoretical minimum work required to compress the gas from suction to discharge pressure relative to the work actually done. The theoretical minimum is based on an idealized ‘isentropic’ process in which no turbulence generation or viscosit y effects occur. The third measure of compressor performance qualit y is the mechanical efficiency. This is the ratio of the work actually done on the gas relative to the measured power input to the compressor shaft. The difference bet ween these t wo figures is due to friction. V a r i a b l e P i s t o n C o m p r e s s o r s As st ated above, Harrison (now <strong>Delphi</strong>) was the first to go into series production with a variable compressor. The wobble plate that drives the pistons with this design is free to tilt so that the piston stroke, and hence displacement can be changed. The wobble plate mechanism allows ver y short and compact pistons to be used. The ball-ended connecting rods are swaged into the pistons and wobble plate which results in strong , low friction and ver y simple joints. The fact that the connecting rods are tilted slightly during compression as a result of the wobble plate movement means that the forces acting do not all lie along the piston axes. The result is that higher vibrational harmonics can be generated which makes management of compressor noise more of a challenge than with later generations of swash-plate compressor. Nonetheless the value of this configuration is illustrated by the fact that it is still produced at a rate of several million units per year worldwide for inst allation in new vehicles as original equipment, offering a good compromise bet ween cost and the functionalit y of a variable displacement compressor. The next generation of variable compressors used a swash-plate mechanism, largely because of its advant ages of low vibration and noise. The mechanism by which the compressor displacement is controlled can be described in principle relatively simply, but in fact the actual displacement of the compressor in any given situation results from a quite complex interaction of forces.
Page 1: Air Conditioning Compressors
Page 4 and 5: e q u i r e d c o o l i n g i s t h
Page 8 and 9: Va r i a b l e s wa sh-plate compre
Page 10 and 11: A C o m p r e s s o r R a n g e One
Page 12: Delphi Thermal Systems European Hea

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